Generally, in a multi-carrier mobile communication system, a base station transmits downlink data packets to user equipments (UEs) belonging to each of at least one or more cells.
A plurality of user equipments can exist within a cell. Each of the user equipments is incapable of knowing when a data packet will be received or what kind of data packet will be received. So, when a base station transmits a downlink data packet to a specific user equipment (hereinafter abbreviated UE), such necessary information as an identification (ID) of the UE to receive the corresponding data packet, a time-frequency domain for transmitting the data packet, a data transport format including coding rate, modulation scheme and the like, HARQ associated information, and the like should be transmitted in downlink for each downlink data packet transmission.
On the contrary, in order to enable a USE to transmit such necessary information as a data packet in uplink, a base station should transmit ID of a UE to be allowed for data packet transmission, uplink time-frequency domain for the UE to transmit the data packet, data transport format including coding rate, modulation scheme and the like, HARQ associated information, and the like in downlink for each uplink data packet transmission.
In case of uplink data packet transmission, a base station should transmit acknowledgement/non-acknowledgement (ACK/NAK) information on each packet transmitted by a UE to the corresponding UE in downlink. Moreover, the base station should transmit power control information to each UE in downlink to sustain uplink transmission/reception power of the corresponding UE at a proper level.
For convenience of explanation, in the following description, every information transferred via physical layers for data transmission and reception between base station and UE is called ‘downlink control information’. And, a signal for carrying this information is called ‘downlink control signal’.
In detail, downlink control information can be categorized as follows.
1. Scheduling Information on Uplink/Downlink Data                (1) Category-A Information: ID of UE to transmit/receive data packets, allocation information on frequency-time domain for carrying data packets, etc.        (2) Category-B Information: Data transport format such as coding rate, modulation scheme and the like, HARQ associated information, etc.        
2. Information Not Associated with Downlink Data                (1) ACK/NAK Information, Power Control Information, etc.        
In order to operate a system efficiently, it is necessary to efficiently multiplex a downlink control signal for carrying the above-mentioned control informations with data packets and other downlink signals in downlink time-frequency resources.
For this, a general downlink signal transmitting system is explained as follows.
First of all, a downlink data packet transmitting system can be mainly categorized into localized allocation and distributed allocation.
In the localized allocation, data for a single UE are transmitted within a relatively restricted frequency banc via consecutive subcarriers. A base station scheduler selects a band having a good radio channel frequency response for each UE based on a frequency response reported by UEs within a cell on a downlink radio channel and then transmits data. Hence, cell transmission efficiency can be raised. For reference, in the localized allocation, a base station is able to transmit data to a single UE via subcarriers within at least two bands discontinuous on frequency if necessary.
In the distributed allocation, data for a single UE is transmitted via a relatively wide frequency band within a system band by being intentionally distributed. The distributed allocation is available for a case that a base station scheduler has difficulty in estimating a downlink radio channel frequency response for UE or applying a frequency response to downlink data packet scheduling. Since a single data packet is transmitted via wide frequency band, frequency diversity gain is obtained. Hence, data packet receiving performance can be enhanced.
In the following description, a downlink signal transmitting system supports both localized allocation and distributed allocation for downlink data packet transmission. And, it is assumed that data packets transmitted differently within a single transmission time can be multiplexed together.
For clarity of the following description, a basic time-frequency domain unit for data packet transmission is named a resource block (hereinafter abbreviated RB). And, it is assumed that a single RB includes a plurality of subcarrier areas across a plurality of OFDM symbols.
FIG. 1 is a diagram of a structure for transmitting a downlink signal by localized allocation.
Referring to FIG. 1, if 288 subcarriers used for downlink data transmission exist within a system band and if a single RB includes 12 subcarriers through 6 OFDM symbols, 24 RBs exist for every 6 OFDM symbols in downlink. In this case, assuming that RB for localized allocation is generally named LVRB (localized virtual RB), the LVRB, as shown in FIG. 1, is constructed with 12 consecutive subcarriers. Looking into an example that LVRBs are allocated to a UE1 in FIG. 1, localized allocation can be implemented in a manner of transmitting data via LVRBs consecutive for a single UE.
Moreover, looking into an example that LVRBs are allocated to UE2, data are transmitted to a single UE via LVRBs spaced apart from each other on a frequency domain. Hence, effect of distributed allocation is consequently implemented to obtain frequency diversity gain.
FIG. 2A and FIG. 2B are diagrams of a structure for transmitting downlink signals by distributed allocation.
Unlike FIG. 1, if RB for distributed allocation is named DVRB (distributed virtual RB), the DVRB, as shown in FIG. 2A or FIG. 2B, can be constructed with subcarriers separated from each other on a frequency domain or a time-frequency domain. In this case, referring to FIG. 2A and FIG. 2B, even if data amounting to a single RB or a small count of RBs are transmitted to a single UE, it is transmitted by being distributed across a wide band through discontinuous subcarriers using DVRBs. Hence, distributed allocation can be implemented.